1. Field of the Invention
This invention relates to a plastic housing structure, a process for producing the same, and a mold structure used in this process. Such a plastic housing structure is used for home electric or electronic units, toys, telephones, office automation machines, and the like, and more particularly is suitable for portable personal computers portable wordprocessors, or the like.
At present, portable units, in particular, notebook type personal computers, or portable wordprocessors have been made lighter and lighter in weight. It should be noted that the material of the housing structure of these machines has been changed from a heavy metal to a plastic material which is lighter in weight and of a reduced specific gravity. However, in these products, the weight of housing is still about one third of the total weight of the product. Thus, it has been required to reduce the wight of the housing structure.
2. Description of the Related Art
A conventionally known plastic housing for notebook type personal computers or portable wordprocessors has been required to be strong and easy to form. In this respect, it has been desired that the thickness of the housing be as uniform as possible, i.e., the thickness being 3 to 5 mm, or about 2.5 mm at a minimum. The specific gravity of a plastic material is generally 0.95 to 2.0, and the specific gravity of a plastic material of high strength is about 1.1 to 1.7. Therefore, it would seem impossible to reduce the weight i.e., the specific gravity, of the products by reducing the weight of housing material. One approach to this problem is to reduce the thickness of the housing walls. However, with this solution there arises the following problems in the prior art technique for forming plastic housings.
Thus, the following problems would arise if the thickness of the housing wall was reduced to 1.0 to 0.7 mm.
1) In general, the deformation of a plate due to a bending stress varies by amount inversely proportional to the thickness squared of the plate. Therefore, if the thickness of a plate is reduced by one-half (1/2), the deformation thereof will be increased four times. In such a manner, if the thickness of a plate is reduced, it becomes too easily deformable and, thus, the housing would become too weak to protect the contents thereof.
2) If the thickness of the housing wall is reduced, when the housing is made by injection molding, the melted resin must flow into a cavity which has the thin, wide area. Thus, a resistance to flow becomes very high and the pressure necessary for flowing the melted resin becomes also high. Therefore, an injection molding machine having a large injection capacity must be used. Also, if the pressure is to be thus increased, a large force is exerted on the mold to open the same, so that a variation of thickness of the housing wall might occur and fins might be generated. In addition, if the thickness of a housing wall is thin, the heated, molten resin comes into contact widely with the inner wall of the mold cavity, when flowing therein, so that the resin is rapidly cooled and thus the temperature of the resin is lowered. This means that the viscosity of the resin is increased and thus a part of resin might become solid before having reached to the terminal portions of the mold product which causes a so-called "short shot".
3) In order to prevent the above-mentioned variations and "short shot", if the thickness of the housing wall is increased at peripheral portions thereof and if the product is provided with ribs which serve as passages for the melted resin when forming the same by molding, the following secondary problems will arise. Namely, the resin will first flow into the portions of low resistance, i.e., the peripheral portions and the ribs, so that gas which could not be discharged to the outside, accummulates at a bottom wall and thin wall portions of the housing and, therefore, a so-called "burned mark (spot)" will arise due to "short shot" or a compression of gas.
4) In a final discharge process for taking the product out of the metal mold, if the product has a thin wall portion and if the product is pushed by a pusher engaging the thin wall portion thereof in the same manner as a general product, the thin wall portion may be damaged or deformed by the pusher, since such a portion has not been sufficiently solidified yet.